Conventional vehicle fuel tank systems have a removable fuel tank cap for sealing the unburned hydrocarbon fuel in the fuel tank system. In general, the fuel tank cap contains pressure relief and vacuum relief valves for relieving excessive pressure or vacuum. However, if the cap is improperly installed or is not installed at all, fuel vapors may escape into the atmosphere due to even slight pressure buildup in the fuel tank. Recent regulation related to the emission of unburned hydrocarbon fuel has required a solution to this problem.
One approach to preventing fuel vapor discharge out of the fuel tank is to use an onboard diagnostic leak detection system. In such a system, a vacuum is applied to the fuel tank and the corresponding fuel tank pressure is measured. If a leak is present, a corresponding pressure response is observed. When no leak is preset, a different pressure response is observed. Thus, it is possible to diagnose a possible fuel tank leak, or an improperly installed fuel tank cap.
Another approach, which may combined with the previous approach, is to use a flap valve, spring biased in the closed position, in the fuel tank filler tube. This allows the fuel tank filler tube to be sealed if the gas tank cap is not properly installed. This system uses hardware to guarantee a physical seal regardless of the installation of the cap. In addition, a valve element is provided that provides pressure equalization across the flap valve when the cap is properly installed. This is necessary because when refueling, a small amount of residual fuel retained in the refueling nozzle will enter the fuel tank filler tube before the flap valve opened. Thus, without the valve element, fuel may spray back on the nozzle operator due to pressure buildup in the fuel tank system. Such a system is disclosed in U.S. Pat. No. 5,595,222.
The inventor herein has recognized a disadvantage with the above approach. For example, the system still relies on the fuel tank cap to allow pressure equalization. Thus, if the fuel tank cap is improperly installed, while no vapors in the fuel system will be emitted, residual fuel in the refueling nozzle may be sprayed on the nozzle operator due to unreleased pressure in the fuel tank system. Another example is that the system still relies on the fuel tank cap to provide a seal when properly installed. In other words, a cap may be properly installed; however, if defective, it may create a poor seal, which can lead to a leak.